Carbonatites are rare carbonate-dominated mantle-derived rocks that, in some cases, host economically significant deposits of REE, Nb, Zr, which are important for modern high-tech and “green” industry. More than 527 carbonatite occurrences have been recorded in the world so far . However, only about 30 mineralized carbonatites host economic resources of REE and Nb [2, 3]. The scientific and practical interest in carbonatites is great, but the processes of carbonatite formation and ore-grade enrichments of Nb and REE are far from being fully understood. Contemporary models of carbonatite petrogenesis fall into three major groups:
(1) primary origin of carbonatitic magmas by partial melting of a carbonated mantle source [4–15];
(2) derivative origin from an homogeneous alkaline silicate magma by silicate-carbonate liquid immiscibility [16, 17];
(3) derivative origin by extensive fractional crystallization of a carbonated silicate parental liquid [18, 19].
Identification of the exact process (or processes) responsible for the formation of carbonatitic magmas is hampered by widespread Na–K metasomatism (fenitization) associated with carbonatites, which manifests the loss of alkalis and volatiles from their parental magmas . In addition, many, if not most, intrusive carbonatites are cumulates, dominated by calcite or dolomite, differing in composition from their parental magma  and often affected by extensive textural and chemical re-equilibration after emplacement .
As seen from experimental works [14, 23–25], direct smelting from the mantle source and silicate-carbonate immiscibility are unable to provide concentration of REE to economically significant levels. Immiscibility experiments in various silicate–carbonate systems have demonstrated that Pb, Nb, Th, U and most of the REE preferentially partition into the silicate liquid, whereas Sr, Ba and F partition into the conjugate liquid carbonate phase [23, 24, 26]. This pattern of element partitioning is inconsistent with a high content of primary LREE- and HFSE-rich minerals in carbonatites: fluorapatite, fluorcarbonates, monazite, pyrochlore, etc. Thus, another evolutionary mechanism is needed for the enrichment of carbonatitic magmas in REE and HFSE, and fractional crystallization might be powerful driver for it.
The majority of experimental estimates of the partition coefficients for REE have been obtained for the main rock-forming minerals (olivine, pyroxene, garnet, amphibole and biotite), and some accessory minerals (such as rutile, apatite, perovskite, and baddeleyite) at P–T conditions of the upper mantle [25, 27–37]. Experimental constraints on REE partitioning in carbonatitic magmas at crustal pressures are sparse. Because of low crystallization temperature and low viscosity of carbonatitic magmas, fractional crystallization can proceed in them up to shallow depths and control the distribution of REE and HFSE, leading to the formation of a deposit or a barren carbonatite body. Therefore, it is important to assess the effect of fractional crystallization at crustal conditions and to identify the influencing factors.
Carbonate minerals are the principal constituents of intrusive carbonatites: their content ranges from 50 modal %, which is accepted as a nominal threshold for this rock type , to well over 90 % in some varieties interpreted as cumulates . Considering the high propensity of calcite to various postmagmatic changes, plasticity and recrystallization, observed trace element contents may be far from the original magmatic pattern. Hence, experimental modelling of trace element distribution between calcite and carbonatite melt at magmatic conditions is important.
Additional ligands such as F−, OH−, Cl−, SO42− and PO43− are likely to have strong effects upon the REE partitioning. In our previous work  we carried out a pilot series of experiments to obtain distribution coefficients for REE and HFSE between calcite, fluorite and nominally dry carbonatitic melts in synthetic compositions with CaO at 37–54 wt.%, Na2O 7–24 wt.%, F 5–9 wt.%, P2O5 between 0 and 9.5 wt.% at 650–900°C and 100 MPa. The distribution coefficients for individual REE in calcite in that study did not exceed 0.1 and in fluorite they were below 0.25. Total REE concentrations in calcites were at about 500–700 ppm. Such concentrations are normal for primary igneous calcites in carbonatites . However, there are described primary igneous calcites with REE contents at 1400–2000 ppm (e.g., the Aley complex, Canada ). This study was aimed at a more detailed assessment of the effects of melt composition, and especially the concentrations of P2O5, SiO2 and SO3 components on the REE distribution between calcite and melt at 900 − 650°C and 100 MPa.